Interpretive Summary: Foodborne pathogens of concern to the beef industry include: E. coli O157:H7, which has been implicated in foodborne illness outbreaks associated with ground beef; Salmonella, which is used as an index of sanitation in ground beef processing plants; and other E. coli, called STEC, which can produce the same toxins as E. coli O157:H7 and may cause disease. Human illnesses associated with these pathogens tend to peak during warmer months. Thus, the objective of this study was to determine the seasonal prevalence of these pathogens in beef processing plants by testing feces, hides, carcasses after the hide was removed but before any washes (pre-wash), and carcasses at the end of processing after all antimicrobial treatments (post-wash) were applied. E. coli O157:H7, Salmonella, and STEC were found more often on hides than in feces. E. coli O157:H7 prevalence in feces peaked in the summer, whereas prevalence on hides was higher in spring, summer, and fall than in winter. Of post-wash carcasses, only 1.2% were contaminated with E. coli O157:H7, all at very low levels. Salmonella prevalence peaked in feces in summer and was highest on hides and pre-wash carcasses in summer and fall. Only one post-wash carcass was contaminated with Salmonella (0.1%). STEC prevalence in feces peaked in spring and fall, and on hides it peaked in fall. These data suggest that: (1) pathogen prevalence varies by season and is generally higher during warm months, (2) the levels and incidence of pathogens are reduced dramatically after all antimicrobial treatments, and (3) effective treatments to reduce pathogens on hides could further reduce the incidence of beef contamination. Finally, these results demonstrate the effectiveness of the current interventions used by the beef processing industry and highlight the significance of hides as a major source of pathogens on beef carcasses.

Technical Abstract:
The seasonal prevalence of Escherichia coli O157:H7, Salmonella spp., non-O157 Shiga toxin-producing E. coli (STEC), and stx-harboring cells was monitored at three Midwestern fed-beef processing plants. Overall, E. coli O157:H7 were recovered from 5.9% of fecal samples, 60.6% of hide samples, and 26.7% of carcasses sampled before pre-evisceration wash. They also were recovered from 1.2% (15 of 1232) of carcasses sampled at chilling (post-intervention), at approximate levels of < 3.0 cells per 100 cm**2. In one case, E. coli O157:H7 levels dropped from ~1100 per 320 cm**2 at pre-evisceration to undetectable on ~2500 cm**2 at post-intervention. E. coli O157:H7 prevalence in feces peaked in the summer, whereas prevalence on hide was high from spring through fall. Overall, Salmonella were recovered from 4.4%, 71.0%, and 12.7% of fecal, hide, and pre-evisceration carcass samples, respectively. Salmonella were recovered from one carcass (of 1016) post-intervention. Salmonella prevalence peaked in feces in summer and was highest on hide and pre-evisceration carcasses in summer and fall. Non-O157 STEC prevalence also appeared to vary by season, but the efficiency of recovering isolates from stx-positive samples ranged from 37.5% to 83.8% and could have influenced these results. Cells harboring stx genes were detected by PCR in 34.3%, 92.0%, 96.6%, and 16.2% of fecal, hide, pre-evisceration carcass, and post-intervention carcass samples, respectively. The approximate level of non-O157 STEC/stx-harboring cells on post intervention carcasses exceeded < 3.0 cells per 100 cm**2 for only 8 of 199 carcasses (4.0%). Overall, the prevalence of E. coli O157:H7, Salmonella, and non-O157 STEC was: (1) variable by season, (2) higher on hides than feces, and (3) reduced dramatically, along with pathogen levels, during processing and the application of antimicrobial interventions. These results demonstrate the effectiveness of the current interventions used by the industry and highlight the significance of hides as a major source of pathogens on beef carcasses.